1. Field of the Invention
This invention relates to a process for producing a polyarylene sulfide (for example, a polyphenylene sulfide) having a low content of heavy metals, excellent electrical characteristics, and excellent heat stability.
Polyarylene sulfide, having excellent heat resistance and excellent chemical resistance, has attracted special attention for use in electronic components and automotive parts. Being moldable into engineering parts made of plastics, films, sheets, fibers, etc., polyarylene sulfide is widely used in fields where heat resistance is needed.
2. Description of the Prior Art
Japanese Patent Publication No. 3368/1970 discloses a process for producing polyarylene sulfide wherein the reaction between p-dichlorobenzene and sodium sulfide is conducted in an organic polar solvent such as N-methyl-2-pyrrolidone or the like. Sodium sulfide containing 2.8 moles or 9 moles of water of hydration per mole of Na.sub.2 S is heated to free the water of hydration under reduced pressure or in N-methyl-2-pyrrolidone solvent with the introduction of bubbles of nitrogen and then p-dichlorobenzene is added thereto, followed by thermal polymerization.
This process, however, causes thee corrosion of reaction vessels made of iron or stainless steel, because highly corrosive sodium sulfide is maintained at high temperatures in the dehydration stage, resulting in elution of heavy metals such as iron or the like into sodium sulfide. Therefore, a significant amount of heavy metals remains in the polymer produced by the reaction with p-dichlorobenzene, which unfavorably causes a reduction in heat stability and electrical characteristics. It is also conceivable to use titanium, chromium, molybdenum, or tungsten, or stainless steels having a high content of these metals, as the constituent materials of a reaction vessel in order to prevent heavy metals remaining in the polymer. However, these materials are undesirable because of their low reliability at high temperatures and pressure and their high cost.